This invention relates generally to semiconductor products and to methods for testing semiconductor products.
As device features and dimensions of integrated circuits continue to shrink, and increase in speed, there is a trend toward using flip chip technology when packaging complex high speed integrated circuits. In flip chip technology, also known as control collapse chip connection (C4) packaging, the integrated circuit is flipped upside down.
It is often necessary to probe electrical signals from internal nodes of the integrated circuit packaged in a flip chip or C4, packaging environment. For example, during the testing, debug or failure analysis process, internal nodes may be probed to obtain significant electrical and timing data from the integrated circuit. Such data include device parameters such as voltage levels, timing information, current levels and thermal information.
Access to the metal interconnects in an integrated circuit may be obstructed by the package substrate. The P-N junctions forming active and passive regions of an integrated circuit may be accessible, however, through the back side of the silicon substrate of an integrated circuit die using an infrared laser voltage probing system such as those offered by NPTest Inc. With such an infrared laser voltage probing system, a laser beam may be directed through the back side of the device under test (DUT), and reflected back to a detector. The detector detects waveforms which depend on the refractive index of different regions in the substrate. Detection is possible due to the plasma-optical effect in which the refractive index of a region of free charge is different from a region with no charge.
The nodes may be one or more probe points selected in an integrated circuit, onto which the infrared laser may be directed. For example, N-type diffusion devices, i.e., transistors, have been used as probe points. An N-type diffusion probe point may be an N-type impurity, i.e., a region having excess electron carriers, implanted within a P-type region in a substrate. N-type diffusion probe points may provide static protection during fabrication of an integrated circuit and typically use very small space on an integrated circuit.
However, as integrated circuits shrink, and voltages in those devices decrease, N-type diffusion probe points are less effective or ineffective for use with infrared laser voltage probing systems. For example, signal to noise ratios may be inadequate, particularly in circuits having small device geometries and/or low voltages, i.e., below about 1.5, volts.
What is needed is a probe point that is more effective for use with infrared laser voltage probing of integrated circuits. A method of testing integrated circuits with infrared laser voltage probing systems also is needed to provide better signal to noise ratios, for increasingly small geometry integrated circuits and low voltage devices.